v0.2.0: Separate branch history by repository

Instead of having a global flat list where all branches are stored from
every repository where `git-checkout-store` is used, separate branch
history so that running `git-checkout-history` will only show the
history for the current repostory you're operating on.

This changes the storage layout of the history database file, so if one
already exists it will have to be deleted or moved.

v0.1.0: First more functional version

Offers a more pleasant experience over v0.0.1.

* Prints `git checkout` output on the screen for confirmation of having
  switched branches
* `git-checkout-store` takes an arbitrary number of arguments and passes
  them to `git checkout`.